Sterols useable in pharmaceuticals and foods need to be purified to quite a high degree to exclude undesired components, such as inorganic salts and soap residues existing in the neutral substances. Although usually present in low concentrations, a part of these impurities usually tend to remain in the sterol fraction, if the isolation of sterols has been performed by known straight crystallization processes without any particular purification steps. The neutral substances in most cases also contain organic impurities, which usually makes direct crystallization of sterols in pure form difficult. One kind of impurity is a group of compounds which typically cannot be seen in gas chromatographic analyses usually used in sterol assays. The exact nature of these compounds is not known, but there is some evidence that this group consists of a wide molecular weight range hydrocarbon-type material. Because of its invisibility in gas chromatographic analysis, the material is often called “non-elutable compounds” or “nonelutables”. Also the lighter components of the neutral substances may sometimes interfere with the crystallization of sterols. In a typical case, when the neutral substances are originated from wood pulping, this light fraction typically consists of impurities such as di- and sesquiterpene compounds, stilbenes and wax alcohols.
Sterols have previously been crystallized from sterol containing materials, typically from neutral substances, using alcohol, ketone or hydrocarbon solvents, without or with water (U.S. Pat. Nos. 2,704,764, 2,729,655, 2,843,610, 5,117,016, 4,420,427). None of these methods work quite satisfactorily in the most usual cases where the substance that contains sterols also contains the impurities noted above.
U.S. Pat. No. 2,870,176 discloses a method of obtaining stigmasterol from a phytosterol solution in which a phytosterol sample is dissolved in hexane and a stigmasterol-enhanced fraction is crystallized from the cooled solution. After separation from the hexane solvent, the stigmasterol-enhanced fraction is redissolved in hexane and recrystallized several times, to obtain substantially pure stigmasterol crystals. There is no disclosure of a step whereby any impurities are separated from the phytosterol fraction.
U.S. Pat. No. 3,965,085 discloses a method for extracting neutral substances from soaps in which a hydrocarbon solvent is used to extract the neutral substances, and thereafter the neutral substance solution phase is evaporated whereby a residue is obtained containing mainly sterols, terpene alcohols, hydrocarbons and other unsaponifiable substances. This solution is cooled to obtain crystalline material. The crystalline material contains sterols, along with fatty alcohols and terpene alcohols. There is no disclosure of a purification method for purifying the sterol crystals. In addition the method does not include any evaporation fractionation step.
U.S. Pat. No. 4,076,700 discloses a process for recovering fatty acids and/or rosin acids and optionally sterols from a tall oil skimming soap or a tall oil soap. According to one embodiment of this process, the soap is first saponified with an alkali to decompose esters of rosin acid and fatty acid with sterols and other alcohols, and the thus obtained saponification product is subsequently introduced into a thin film evaporator to evaporate and remove water and low boiling unsaponifiables. The product of this stage can again be introduced into the thin film evaporator to evaporate and separate sterols and heavy unsaponifiables. For the production of pure sterols, this proposed process is very complicated. In addition, this proposed process does not use the technique of extracting neutral substances from soaps.